The well known concept of the organization of cell materials and functions into compartments has logically been extended to the organization of certain enzymes which carry out some of these functions. Extensive ordering of some enzymes into multifunctional complexes has been shown. These enzymes are particularly interesting because of the increased efficiency with which they yield a final product. In the histidine operon, which specifies the synthesis of the histidine biosynthetic enzymes, the hisB gene of Salmonella typhimurium controls two of the ten biosynthetic events. Therefore, hisB is a bifunctional gene and, consequently, the hisB enzyme is also bifunctional. Imidazole glycerolphosphate dehydratase-histidinol phosphate phosphatase, the hisB enzyme, will be purified from a derepressed mutant and studied to determine its molecular size and its subunit molecular weight. The structure of the enzyme will also be studied by the use of mutants which define various subregions within hisB. It is hoped that some conditions, either of isolation or reconstitution, will be found so that a larger multifunctional complex of several of the histidine enzymes can be shown. It is hoped that these studies will provide (1) an understanding of the hisB gene product, particularly the nature of its dual specificity; (2) evidence that the histidine biosynthetic enzymes exist in an aggregated state in the cell; (3) detailed information that contrasts the control of individual enzymes in the dissociated form with their control in the integrated complex; (4) some evidence as to the evolutionary history of these enzymes and (5) extension of the idea of the flexibility of enzymes - that has been so useful in the explanation of enzyme mechanism (induced fit) and allosterism - to an even more complex level, that of the multienzyme aggregate. In addition, a variety of techniques will be used to look at the individual sites of phosphatase and dehydratase action in order to be able to differentiate them and study the sites more or less independently. Considerable effort will be made to understand the physical relationships of the two activities and to gain insights into the "fusing" of enzyme activities and their evolutionary advantages (and disadvantages) over more "normal" enzymes.